(a) Field of the Invention
This invention relates to a pulse-width modulation circuit which may be used for the effective amplification of such as audio signals, and more particularly it relates to a pulse-width modulation circuit which is independent of and free from the non-linearity of a waveform of a carrier signal.
(b) Description of the Prior Art
It is known in the art that an audio signal amplifier can obtain an extra high efficiency by associating a pulse-width modulation circuit therewith. FIG. 1 shows a circuit diagram of one example of such conventional audio signal amplifiers. In the figure, a pulse-width modulation circuit designated by a reference number 1 converts an audio signal to be amplified into a pulse signal having a duty ratio corresponding to the amplitude of the audio signal. The pulse-width modulation circuit 1 comprises a comparator 2 having inverting and non-inverting input terminals. The non-inverting input terminal receives an audio signal Ea (voltage signal) to be amplified via an input terminal 3, while the inverting input terminal receives via an input terminal 4 a carrier signal Ec (voltage signal), such as of a triangular wave, a sawtooth wave or the like, which is necessary for effecting a pulse-width modulation. When the audio and carrier signals Ea and Ec having respectively waveforms as shown in FIG. 2(a) are applied to the input terminals of the pulse-width modulation circuit 1, a pulse signal Ep with a waveform as shown in FIG. 2(b) is generated, having an equal period or repetition frequency to that of the carrier signal Ec and a duty ratio varying in correspondence with the amplitude of the audio signal Ea. The pulse signal Ep is then power-amplified in a pulse amplifier 5, and thereafter is subjected to demodulation in a low-pass filter 6 wherein the carrier and audio signals Ec and Ea are discriminated to deliver only the components of the audio signal Ea to a loudspeaker 7.
In the prior art pulse-width modulation circuit 1 of FIG. 1, as readily understood from the illustration of FIG. 2, substantially large distortion may develop unless the slope of the waveform of the carrier signal Ec, such as a triangular wave or a sawtooth wave, varies linearly with time, that is, unless the duty ratio of the pulse signal Ep varies exactly in proportion to the amplitude of the input signal Ea. However, practically, it is very hard to produce a correct linear slope for the triangular and saw-tooth waves.
In order to obviate such a hardship accompanied by the pulse-width modulation circuit described above, another type of a pulse-width modulation circuit has been proposed heretofore, as shown in FIG. 3.
Referring to FIG. 3, a pulse-width modulation circuit 1 comprises an operational amplifier 8, a comparator 9, and a pulse amplifier 10. An audio signal Ea is supplied through an input terminal 3 to a non-inverting input terminal of the operational amplifier 8, and a carrier signal Ec of a square wave is supplied through an input terminal 4 and a resistor 11 to an inverting input terminal of the operational amplifier 8. The operational amplifier 8, of a Miller integration circuit arrangement with the resistor 11 and a capacitor 12, integrates the carrier signal Ec (square wave) to generate a triangular wave, the audio signal Ea being added to the triangular wave. As a result, an output signal E1 of the operational amplifier 8 is generated, having a waveform as exemplarily illustrated in FIG. 4(a). The signal E1 is then applied to a non-inverting input terminal of the comparator 9 so as to be compared with a ground level. The comparator 9 outputs a pulse signal Ep as of FIG. 4(b), the pulse signal having an equal period to that of the carrier signal Ec and a duty ratio corresponding to the amplitude of the audio signal Ea. The pulse signal Ep, after being subjected to non-inverting amplification in the pulse amplifier 10, is negative-fed back to the inverting input terminal of the operational amplifier 8 through a resistor 13. The output of the pulse-width modulation circuit 1, and hence the output of the pulse amplifier 10, is demodulated by a low-pass filter 6 to drive a loudspeaker 7.
As appreciated from the pulse-width modulation circuit 1 of FIG. 3, a negative feedback is given to a portion of the integrated carrier signal (triangular wave) and also to the audio signal Ea, so that a modulated output with lesser distortion can be obtained than the pulse-width modulation circuit 1 of FIG. 1. The carrier signal Ec is applied to the pulse-width modulation circuit 1 of FIG. 3, however, can not share in the benefit of a negative feedback. More in particular in this case, if the carrier signal Ec (square signal) itself has distortions resulting from such as residual noises, distorted square waveforms, or symmetrical amplitudes, this kind of distortions are amplified and developed at the output of the pulse-width modulation circuit 1. Consequently, it is necessary to provide such a carrier signal Ec as having no distortion. However, it is most difficult to fabricate an oscillator circuit generating a distortionless carrier signal.